This invention relates to a fuel cell system and more particularly to a system having a plurality of cells which consume an H2-rich gas to produce power.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors may include a water/gas shift (WGS) and preferential oxidizer (PROX) reactors. In the PROX, carbon dioxide (CO2) is produced from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in U.S. Pat. Nos. 6,232,005, 6,077,620, and 6,238,815, issued respectively May 15, 2001, Jun. 20, 2000, and May 29, 2001, assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, assigned to General Motors Corporation, and incorporated herein by reference in their entirety.
The electrically conductive elements sandwiching the MEAs may contain an array of grooves in the faces thereof for distributing the fuel cell""s gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. In the fuel cell stack, a plurality of cells are stacked together in electrical series while being separated one from the next by a gas impermeable, electrically conductive bipolar plate. Heretofore, the bipolar plate has served several functions including (1) as an electrically conductive gas separator element between two adjacent cells; (2) to distribute reactant gases across substantially the entire surface of the membrane; (3) to conduct electrical current between the anode of one cell and the cathode of the next adjacent cell in the stack; (4) to keep the reactant gases separated in order to prevent auto ignition; (5) to provide a support for the proton exchange membrane; and (6) in most cases to provide internal cooling passages therein defined by internal heat exchange faces and through which a coolant flows to remove heat from the stack. The bipolar plate also accommodates the gas pressure loads as well as the compression loads on the plates. For example, the plate includes a plurality of channels on one side and a plurality of channels on the other side with the channels on an individual side being separated by lands. The arrangement of the lands and the channels on both sides has to be such that the bipolar plate can withstand the compression loads so that the lands and the channels are arranged so that they do not collapse or warp the bipolar plate. The bipolar plate includes channels to deliver the hydrogen and oxygen to a proton exchange membrane assembly overlying the bipolar plates. A piece of graphite paper is placed over the serpentine channels to prevent the membrane from collapsing down into the channel and blocking the flow of gas and to provide an electrical conduction path to the bipolar plate from the area of the membrane which overlays the channel.
The bipolar plates may be made from metal but the plates can also be manufactured from other materials. For example, bipolar plates are often fabricated from graphite which is lightweight (compared to traditional metal plates), corrosion resistant and electrically conductive in the PEM fuel cell environment. However, graphite is quite brittle which makes it difficult to handle mechanically and has a relatively low electrical and thermal conductivity compared to metals. Finally, graphite is quite porous making it virtually impossible to make very thin gas impervious plates which is desired for low weight, low volume, low internal resistant fuel cell stacks.
Neutzler, U.S. Pat. No. 5,776,624, discloses a metal bipolar plate and PEM assembly of this channel type. These prior art bipolar plates and PEM assemblies are heavy, bulky, difficult to produce and assemble, and costly to manufacture.
By contrast, efficient operation of a fuel cell system depends on the ability of the fuel cell to generate a significant amount of electrical energy for a given size, weight, and cost of the fuel cell. Maximizing the electrical energy output of the fuel cell for a given size, weight, and cost is especially important in motor vehicle applications where size, weight, and cost of all vehicular components are especially critical to the efficient manufacture and operation of the vehicle. Therefore it is desirable, especially for motor vehicle applications, to provide a fuel cell construction which will generate an increased amount of electrical energy for a given size, weight, and cost of the fuel cell.
The invention relates to a proton exchange membrane fuel cell including a membrane electrode assembly (MEA) comprising a proton transmissive membrane, a catalytic anode layer on one face of the membrane, and a catalytic cathode layer on the other face of the membrane, and an electrically conductive distribution layer on each of the cathode and anode layers defining a gas flow field extending over each of the catalytic layers.
According to the invention, the MEA has a convoluted configuration. This arrangement has the affect of increasing the ratio of membrane area to effective planar area of the fuel cell whereby to increase the electrical output of the fuel cell for a given effective planar fuel cell area.
According to a further feature of the invention, each gas distribution layer defines a convoluted surface juxtaposed to the respective catalytic layer. This arrangement maximizes the contact interface between the MEA and the gas distribution layers whereby to further increase the electrical output of the cell per unit of planar area.
According to a further feature of the invention, the surface of each gas distribution layer opposite the convoluted surface is generally planar. This arrangement facilitates the stacking of individual cells to form a fuel cell stack.
According to a further feature of the invention, each gas distribution layer is formed of a conductive porous media. This arrangement facilitates the delivery of the respective gases to the respective catalytic layers.
According to a further feature of the invention, the porous media comprises a foam media. This arrangement allows the use of readily available, relatively inexpensive foam material to provide the porous media. In the disclosed embodiment of the invention, the foam media comprises either a conductive graphite foam media or a conductive metallic foam media.
According to a further feature of the invention, the MEA and the gas distribution layers form a sandwich construction having first and second opposite edges; and each gas distribution layer is divided by the convolutions of the MEA into a plurality of generally parallel segments each extending from the first edge to the second of the sandwich construction whereby to define a plurality of generally parallel porous reactant paths extending across each catalytic layer. With this arrangement the gases are confined by the parallel paths for movement in the respective paths so that little or no cross migration occurs between the parallel paths whereby to ensure an essentially uniform distribution of gas across the surface of the respective underlying catalytic layer irrespective of unavoidable and significant variations in the porosity of the foam material of the porous media, whereby to maximize the generation of electrical energy occurring by virtue of the interaction between the gases and the catalytic layers.
According to a further feature of the invention, the fuel cell further includes upper and lower generally planar gas separators defining a space therebetween and the MEA and the gas distribution layers are positioned in the space with the peaks of the MEA positioned proximate the upper gas separator and the valleys of the MEA positioned proximate the lower gas separator. This arrangement accentuates the separation of the porous reactant paths extending across each catalytic layer.
According to a further feature of the invention, each gas distribution layer has an overall convoluted configuration corresponding to the convoluted configuration of the MEA and is positioned in meshing fashion against a respective catalytic layer. This arrangement provides an unobstructed gas flow area across the fuel cell which in turn allows a reduction in the overall size of the fuel cell while remaining within accepted pressure drop specifications.
According to a further feature of the invention, the fuel cell further includes a conductive lower generally planar gas separator and a conductive upper generally planar gas separator positioned above the lower gas separator and defining a space therebetween, and the MEA is positioned in the space with successive peaks on one of the gas distribution layers in electrical contact with successive points on the upper gas separator and successive valleys of the other gas distribution layer in electrical contact with successive points on the lower gas separator. This arrangement provides the required electrical conductivity through the fuel cell while maintaining unobstructed flow of gases across the catalytic layers with consequent reduction in the overall size of the fuel cell.
According to a further feature of the invention, the convolutions of the MEA are uneven such that one of the upper and lower channels are larger than the other of the upper and lower channels. This arrangement facilitates the delivery of disparate quantities of the respective gases to the respective catalytic layers. In the disclosed embodiment of the invention, alternate convolutions are relatively wide, whereby to define the larger channels, and the remaining convolutions are relatively narrow, whereby to define the smaller channels.